CN117411098B - Self-adaptive virtual admittance current limiting method based on voltage drop dynamic - Google Patents
Self-adaptive virtual admittance current limiting method based on voltage drop dynamic Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
- H02J3/48—Controlling the sharing of the in-phase component
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H9/00—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection
- H02H9/02—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection responsive to excess current
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/28—Arrangements for balancing of the load in a network by storage of energy
- H02J3/32—Arrangements for balancing of the load in a network by storage of energy using batteries with converting means
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/40—Synchronising a generator for connection to a network or to another generator
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
- H02J3/466—Scheduling the operation of the generators, e.g. connecting or disconnecting generators to meet a given demand
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
- H02J3/50—Controlling the sharing of the out-of-phase component
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
- H02J2203/10—Power transmission or distribution systems management focussing at grid-level, e.g. load flow analysis, node profile computation, meshed network optimisation, active network management or spinning reserve management
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
- H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
- H02J2300/20—The dispersed energy generation being of renewable origin
- H02J2300/22—The renewable source being solar energy
- H02J2300/24—The renewable source being solar energy of photovoltaic origin
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Abstract
The invention relates to a self-adaptive virtual admittance current limiting method based on voltage drop dynamics, when a grid-connected point fails, a fault detection signal lvrt _flag is set to 1 from 0, the output of a power angle and a virtual potential generation layer are locked to rated values, an active power reference value and a reactive power reference value are calculated and substituted into a quasi-static control mathematical model to obtain a virtual resistor and a virtual inductor, and the obtained virtual resistor and virtual inductor in a current control model are updated; the grid-connected point fault clears and the fault detection signal lvrt _flag is set to 0 by 1. The beneficial effects of the invention are as follows: the synchronous mode in the fault period provided by the invention enables the inverter based on the virtual synchronous generator algorithm to operate in a self-adaptive manner and the power grid to keep synchronous, and the virtual admittance is self-adaptively adjusted according to the voltage drop dynamic so as to improve the power supporting capability of the system, thereby overcoming the defects that the traditional direct freezing synchronous integrator keeps synchronous and the current limiting and weakening stability are realized based on the mode switching mode.
Description
Technical Field
The invention relates to the technical field of grid-connected faults of photovoltaic energy storage virtual synchronous generators, in particular to a self-adaptive virtual admittance current limiting method based on voltage drop dynamics.
Background
The photovoltaic energy storage virtual synchronous generator is widely applied to improving the stability, frequency adjustment and voltage support of a weak power grid, and an energy storage element of the photovoltaic energy storage virtual synchronous generator can be passively stressed to provide inertial support for a system during the fault period of the weak power grid, but the fault crossing capability of the photovoltaic energy storage virtual synchronous generator (PV-ESS-VSG) is weak, and the photovoltaic energy storage virtual synchronous generator is different from the grid synchronization based on a PLL and the fault crossing realization based on limiting reference current during the fault period of a grid inverter, and the fault crossing strategy based on the Virtual Synchronous Generator (VSG) needs to consider how to limit transient current during the fault period and cause the loss of synchronization with the power grid due to the power angle instability under the large disturbance of the power grid.
To solve the above problems, adding a current limiter in a current loop, quickly adjusting parameters such as VSG virtual potential, or switching to a current limiting control mode in combination with a virtual admittance structure is generally adopted to inhibit the problem of over-current of the output of the converter caused by low-voltage faults, however, the off-grid operation of the photovoltaic power station is caused by VSG power angle instability caused by the over-current-based VSG inertia support power out-of-limit, so that a power angle limiter is generally added in a VSG synchronization unit to improve transient stability of the output current of the converter under saturation.
In the invention, based on synchronous generator transient stability concept and method (power angle, power angle characteristic curve and generator excitation state), a method for keeping synchronization during faults and a PV-ESS-VSG low voltage fault ride through strategy for taking voltage drop dynamic self-adaptive current limiting into account are provided, the proposed scheme locks the power angle during faults to prevent the power angle from being unstable, determines a power reference amount during faults according to balanced and unbalanced voltage drop depths and self-adaptively adjusts a virtual admittance value to indirectly limit the output current of an inverter, and in addition, the scheme can automatically provide reactive support for the system during the faults and ensure the stable operation of the PV-ESS-VSG system, thereby being suitable for reliable grid connection of a large-scale photovoltaic power station based on grid-configuration control.
Disclosure of Invention
Aiming at the current limiting strategy during the low-voltage fault ride-through of the existing VSG, the invention provides a self-adaptive virtual admittance current limiting method based on voltage drop dynamics, which combines the characteristic of the power angle and the excitation state of a synchronous generator to prevent the instability of the power angle of the VSG and limits the output current under the condition of providing the required reactive power support for the fault point.
In order to achieve the above purpose, the present invention provides the following technical solutions: a self-adaptive virtual admittance current limiting method based on voltage drop dynamics comprises the following specific steps:
1) When the grid connection point fails, a fault detection signal Will be set to 1 from 0;
2) The power angle and the virtual potential generation layer output are locked to rated values;
3) Calculating an active power reference value and a reactive power reference value, and substituting the active power reference value and the reactive power reference value into a quasi-static control mathematical model to obtain a virtual resistor and a virtual inductor;
4) Updating the virtual resistor and the virtual inductor obtained in the step 3) in the current control model;
5) Grid-connected point fault clearing and fault detection signals Setting 0 from 1;
6) Fault signal The falling edge triggers the reset integrator I 1, and updates the initial value of the reset integrator I 1 to output the electric angle from the synchronous control layer at the moment, and the output of the virtual potential generating layer is unlocked;
7) Repeating the steps 3) and 4), and updating the virtual resistor and the virtual inductor to steady-state values.
Further, the power reference value to be compensated during the fault period in step 1) is expressed as follows:
（1）
Wherein the method comprises the steps of And/>Respectively the active power and reactive power reference values which should be output when in fault,/>And/>For the rated apparent power and its maximum, the maximum takes 1.1 times the rated value. /(I)Grid-connected point voltage per unit value,/>And/>Respectively negative sequence components of grid-connected point voltages.
Further, the formula for calculating the active power reference value and the reactive power reference value in the step 3) is as follows:
（3）
Wherein the method comprises the steps of And/>The virtual resistor and the virtual inductor are respectively used for preventing the switching tube from being damaged due to the fact that the switching tube passes through large current during the transient state of the converter, adapting to different power grids and providing power support, and according to the rule of grid-connected standards, outputting reactive current is prioritized during the fault period, and the reference value of the active current can be calculated through the annular current limiter so as to ensure that the inverter is not excessively flowed;
（4）
In the above And/>Are all current inner loop reference values,/>Output rated current per unit value for positive sequence,/>For the per unit value after grid-connected point voltage drop,/>For the maximum current value that the converter can bear under a certain safety margin, take/>Is that；
Reactive power compensation coefficient of converter during faultAnd/>Is defined as:
（5）
maximum apparent power of network-side converter during fault According to rated mains voltage/>Maximum current that can be transferred/>Adjustment is made, which is expressed as:
（6）
（7）
Wherein the method comprises the steps of For positive sequence rated apparent power, the converter outputs active power when asymmetric voltage drop occurs at the network sideAnd reactive/>The expression is:
（8）
、/> and/> 、/>The active and reactive output values of the VSG and the positive sequence reference value of the VSG are respectively obtained;
Wherein:
（9）
In the formula (8), the amino acid sequence of the compound, Is the direct current component, the secondary cosine component and the secondary sine component of the active power,/>Is a direct current component, a secondary cosine component and a secondary sine component of reactive power,/>，/>Network side voltage and current positive sequence/>, respectivelyShaft sum/>Axis component,/>，/>Negative sequence of network side voltage and current/>, respectivelyShaft sum/>The axial component, positive and negative sequence components are extracted according to a trap, and the transfer function of the trap is as follows:
（10）
In the above Is a differential operator,/>Is the quality factor of the trap, and the value of the quality factor is related to the specific design of the trap effect.
Further, when the grid side fails asymmetrically, the formula (8) can know that the active power and the reactive power injected into the power grid by the converter have double frequency fluctuation, and the 4 input quantities in the formula (9) are difficult to control the 6 output quantities, so that the control method comprises two free quantities, namely, balance current control and power constant control, wherein the balance current control is used for inhibiting negative sequence current components output by the converter and improving the current balance capacity of the injected grid side, the power constant control is used for inhibiting the double frequency fluctuation of the power grid, the reactive power double frequency fluctuation is not considered, and in addition, the negative sequence current command under the asymmetric voltage drop is inhibitedThe power reference value is associated with the d, q axis currents for the purpose of:
（11）
In combination with (3), (7) and (11) virtual admittance during failure can be obtained:
（12）
In the above And/>The updated values of the virtual resistor and the virtual inductor under the current limiting strategy in the step 4);
further can obtain
（13）
As can be seen from the formula (13), the method of injecting positive-order active current and compensating positive-order reactive current based on the voltage drop degree during the fault period actually becomes to directly adjust the magnitude of the virtual admittance, and the method of dynamically adaptively adjusting the virtual admittance can effectively inhibit transient current during the fault period and simultaneously inhibit the impact current caused by switching of the fault modes.
Further, the virtual admittance update value formula in the balance current control is as follows:
（2）
In the middle of And/>Updated values of virtual resistance and virtual inductance under fault period limit respectively,/>And/>The d-axis and q-axis components of the excitation electromotive force,/>, respectivelyAnd/>And the values are the per-unit values of the grid-connected point voltage positive sequence components respectively.
Compared with the prior art, the technical scheme of the application has the following beneficial effects:
(1) The synchronization mode in the fault period provided by the invention can keep the self-adaptive VSG and the power grid to run synchronously, thereby improving the defect that the stability is weakened in the traditional synchronization mode of the direct freezing integrator I 1;
(2) According to the invention, analysis is performed based on the power angle and excitation characteristics of the traditional synchronous generator, and the change of the active reference value during the fault period can improve the stability of the power angle of the converter, and the power angle and the virtual internal potential are locked on the premise that the energy storage system provides inertia support power so as to strengthen the traversing capability of the VSG under grid-connected fault;
(3) The self-adaptive current limiting method provided by the invention can automatically adapt to the voltage drop value without setting an additional fault ride-through mode, so that uncertainty between VSG control output quantity and an actual measured value during mode switching is avoided;
(4) The simulation result shows that the PV-ESS-VSG fault ride-through method taking the dynamic self-adaptive current limiting of voltage drop into account has good ride-through and current limiting capability in balanced and unbalanced voltage drop faults.
Drawings
FIG. 1 is a control schematic diagram of the present invention;
FIG. 2 is a diagram of the VSG synchronization control layer and VSG virtual potential generation of the present invention;
FIG. 3 is a block diagram of an annular restrictor of the present invention;
FIG. 4 is a flow diagram of an adaptive virtual admittance limit of the present invention;
FIG. 5 is a fault state positive and negative sequence control diagram of the present invention;
FIG. 6 is a graph showing the change of power angle in three different synchronization modes according to the present invention;
FIG. 7 is a diagram of a conventional active priority based current limiting method according to the present invention;
FIG. 8 is a diagram of the proposed adaptive current limiting method under the variable active reference of the present invention;
FIG. 9 is a waveform diagram of the active output and DC link voltage of the optical storage VSG of the present invention;
FIG. 10 is a graph of the virtual admittance adaptive modulation results during a voltage sag in accordance with the present invention;
Fig. 11 is a graph of output waveforms of the voltage and the output current of the grid-connected point during voltage sag of the present invention.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
Referring to fig. 1-11, an adaptive virtual admittance current limiting method based on voltage drop dynamics in this embodiment includes the following specific steps:
1) When the grid connection point fails, a fault detection signal Will be set to 1 from 0;
2) The power angle and the virtual potential generation layer output are locked to rated values;
3) Calculating an active power reference value and a reactive power reference value, and substituting the active power reference value and the reactive power reference value into a quasi-static control mathematical model to obtain a virtual resistor and a virtual inductor;
4) Updating the virtual resistor and the virtual inductor obtained in the step 3) in the current control model;
5) Grid-connected point fault clearing and fault detection signals Setting 0 from 1;
6) Fault signal The falling edge triggers the reset integrator I 1, and updates the initial value of the reset integrator I 1 to output the electric angle from the synchronous control layer at the moment, and the output of the virtual potential generating layer is unlocked;
7) Repeating the steps 3) and 4), and updating the virtual resistor and the virtual inductor to steady-state values.
Step 1) the formula of the power reference value to be compensated during the fault is as follows:
（1）
Wherein the method comprises the steps of And/>Respectively the active power and reactive power reference values which should be output when in fault,/>And/>For the rated apparent power and its maximum, the maximum takes 1.1 times the rated value. /(I)The unit values of the grid-connected point voltages, u - gd and u - gq, are the negative sequence components of the grid-connected point voltages respectively.
The virtual admittance update value formula in the balance current control in the step 4) is as follows:
（2）
In the middle of And/>Updated values of virtual resistance and virtual inductance under fault period limit respectively,/>And/>The d-axis and q-axis components of the excitation electromotive force,/>, respectivelyAnd/>Respectively the positive sequence components of the grid-connected point voltage.
In order to prevent the unstable power angle during the fault period from leading the off-grid operation of a photovoltaic power station and the VSG to enter an overexcitation state to lead the overcurrent formed by a reactive ring to be formed when the fault is cleared, the original power angle curve balance mode is changed under the condition of considering current limiting, the power angle during the fault period is required to be kept at a rated value, the active power is regulated slowly during the fault period and the reactive power is required to be compensated rapidly, so that the active reference value is changed only to cause the initial overcurrent of the fault, the invention leads the active ring of the synchronous control ring VSG to output an integrator I 1 and then adds a power angle limiting link, automatically limits the power angle to the rated value to keep the VSG synchronous with the power grid when the fault occurs, and resets the initial value of the integrator I 2 to the output value of the synchronous control layer at the moment when the falling edge of a fault signal is detected。
Reactive power negative feedback introduced by the reactive power loop reduces the virtual electromotive force to avoid overexcitation, but limits the reactive support capability of the inverter during the fault, in order to make the VSG virtual internal potential constant during the fault, the reactive current can be adjusted according to a low voltage ride through standard, the reactive reference value in the reactive power loop is set as the actual reactive power during the fault, and the voltage rated value in the reactive voltage droop loop is set as the actual voltage rated value.
The method is a method for keeping the VSG synchronous with the power grid during the fault period, and is also a technology implementation stage one, and specifically shown in fig. 2.
The first stage is a method for maintaining synchronization of VSG during fault, the second stage also needs to comprehensively consider symmetrical and asymmetrical fault current limiting problems based on the first stage method, and the generation of positive sequence reference current based on quasi-static model simulation virtual admittance is expressed as:
（3）
Wherein the method comprises the steps of And/>The virtual resistor and the virtual inductor are respectively used for preventing the switching tube from being damaged due to the fact that the switching tube passes through large current during the transient state of the converter, adapting to different power grids and providing power support, and according to the rule of grid-connected standards, outputting reactive current is prioritized during the fault period, and the reference value of the active current can be calculated through the annular current limiter so as to ensure that the inverter is not excessively flowed;
In order to prevent the damage of the switching tube caused by the high current passing through the switching tube during the transient state of the converter, adapt the switching tube to different power grids and provide power support, the output reactive current is prioritized during the fault period according to the grid-connected standard, and the reference value of the active current can be calculated through the annular current limiter to ensure that the inverter does not overflow, and the structure of the annular current limiter is shown in figure 3.
（4）
In the aboveAnd/>Are all current inner loop reference values,/>Output rated current per unit value for positive sequence,/>For the per unit value after grid-connected point voltage drop,/>For the maximum current value that the converter can bear under a certain safety margin, take/>Is that；
Reactive power compensation coefficient of converter during faultAnd/>Is defined as:
（5）
maximum apparent power of network-side converter during fault According to the voltage of the power grid/>Maximum current that can be transmittedAdjustment is made, which is expressed as:
（6）
（7）
Wherein the method comprises the steps of For positive sequence rated apparent power, the converter outputs active power when asymmetric voltage drop occurs at the network sideAnd reactive/>The expression is:
（8）
、/> and/> 、/>The active and reactive output values of the VSG and the positive sequence reference value of the VSG are respectively obtained;
Wherein:
（9）
in the formula (9), the amino acid sequence of the compound, Is the direct current component, the secondary cosine component and the secondary sine component of the active power,/>Is a direct current component, a secondary cosine component and a secondary sine component of reactive power,/>，/>Network side voltage and current positive sequence/>, respectivelyShaft sum/>Axis component,/>，/>Negative sequence of network side voltage and current/>, respectivelyShaft sum/>The axial component, positive and negative sequence components are extracted according to a trap, and the transfer function of the trap is as follows:
（10）
In the above Is a differential operator,/>Is the quality factor of the trap, and the value of the quality factor is related to the specific design of the trap effect.
When the grid side fails asymmetrically, the formula (8) can know that the active power and the reactive power of the converter injected into the power grid have double frequency fluctuation, and the 4 input quantities in the formula (9) are difficult to realize the control of 6 output quantities, so that the control method comprises two free quantities, namely two control modes, namely balance current control and power constant control, wherein the balance current control is used for inhibiting the negative sequence current component of the converter output and improving the current balance capacity of the grid side, the power constant control is used for inhibiting the double frequency fluctuation of the power grid power without considering the reactive power double frequency fluctuation, and in addition, the negative sequence current command under the condition of inhibiting the asymmetrical voltage drop is controlled byThe power reference value is associated with the d, q axis currents for the purpose of:
（11）
In combination with (3), (7) and (11) virtual admittance during failure can be obtained:
（12）
In the above And/>The updated values of the virtual resistor and the virtual inductor under the current limiting strategy in the step 4);
further can obtain
（13）
As can be seen from the formula (13), the injection of the positive-sequence active current and the compensation of the positive-sequence reactive current based on the voltage drop degree during the fault period actually becomes direct adjustment of the virtual admittance, and the dynamic adaptive adjustment of the virtual admittance can effectively inhibit transient current during the fault period and inhibit the impact current caused by the switching of the fault modes. In addition, the method is also suitable for symmetrical faults, the power reference value is adaptively adjusted according to the formula (7) under the symmetrical faults, and the specific technical scheme is shown in fig. 4.
Fig. 5 is a block diagram of positive and negative sequence control under a fault, which is an extension of the divided region C of fig. 1, according to the aforementioned crossing method with respect to the fault period. Firstly, respectively collecting grid-connected point voltagesAnd current/>And the positive sequence voltage and the negative sequence current are obtained through the sequence component separation module, the coordinate transformation module and the wave trap in sequence. Negative sequence/>Shaft sum/>The reference current of the shaft is set to 0 to inhibit the system from generating negative sequence current to balance the output current. Finally, generating/>, by means of a quasi-proportional resonant controller (QPRC)Three-phase SPWM modulating the wave and controlling the three-phase bridge to improve the ride-through capability of the PV-ESS-VSG, the transfer function/>, of QPRC is given below。
（14）
In the middle ofIs a proportionality coefficient,/>Is the resonance coefficient,/>Is the damping coefficient.
To verify the effectiveness of the synchronization method and transient over-current suppression strategy during the fault, FIGS. 6-8 show that the grid-connected point voltage drops during 0.5 s-1 sIs a symmetric fault simulation waveform of (1). From fig. 6, it is observed that the virtual power angle is in/>, in the synchronous control of the fixed active reference valueThe virtual power angle is reduced to 6.6rad quickly and then increased slowly by the method for synchronizing the variable active power reference value, and the proposed synchronous control method keeps the power angle in the fault period unchanged, which shows that the variable active power reference value in the fault period can effectively inhibit the instability of the VSG power angle and improves the synchronous capability in the fault period.
In addition, in order to examine the suppression of transient overcurrent during symmetrical faults, the invention respectively carries out simulation test on the traditional current limiting strategy based on active priority and the self-adaptive current limiting strategy proposed under the active reference. As can be seen from fig. 7, the grid-tie point voltage drops to during grid faultsAnd both the d-axis current and the output current peak value of the converter are limited to/>But only a small amount of q-axis current and reactive power is injected into the grid side. Unlike the foregoing, the converter in fig. 8 adaptively compensates more reactive power for the grid. Therefore, the self-adaptive current limiting method provided by the invention can effectively inhibit overcurrent under the condition of providing reactive power support for the system during faults.
To further verify the power balancing capability of the ESS during fault and when the environment changes, the simulation time is divided into three phases for testing, and the voltage drop value is set to be 0.5-1 s in the phase oneThe second stage reduces the irradiation degree by half (halving the output power of the photovoltaic) at 1.5 s-2 s, and the third stage cuts off the photovoltaic (setting the output power of the photovoltaic to 0) at 2.5 s-3 s. FIG. 9 shows that the photovoltaic output during steady state is 1.5/>The output power of the converter and the energy storage is 1/>, respectivelyAnd-0.5/>At the moment, the voltage amplitude of the direct current bus is 1/>(800V). Active power reduction to 0.2 in stage one converterThe energy storage output power and the DC bus voltage amplitude are respectively-1.3/>And 806V, the photovoltaic output in the second stage and the third stage is reduced to 0.75/>, respectivelyAnd 0/>The corresponding energy storage output is 0.25/>, respectivelyAnd 1/>The dc bus voltage magnitudes are 798V and 795V, respectively. Simulation results show that the energy storage system can adaptively provide inertia support power for the VSG to improve the reliability of new energy power generation no matter faults or environmental changes.
FIG. 11 shows the output waveforms of grid-tie point voltage and output current with phase A dropping to 0.2pu, output current during failureIs limited to 1.1/>While the recovery phase output current is increased by a small margin to end/>And the device returns to a steady state, and shows good smooth crossing effect.
The beneficial effects of the invention are as follows:
(1) The synchronization mode in the fault period provided by the invention can keep the self-adaptive VSG and the power grid to run synchronously, thereby improving the defect that the stability is weakened in the traditional synchronization mode of the direct freezing integrator I 1;
(2) According to the invention, analysis is performed based on the power angle and excitation characteristics of the traditional synchronous generator, and the change of the active reference value during the fault period can improve the stability of the power angle of the converter, and the power angle and the virtual internal potential are locked on the premise that the energy storage system provides inertia support power so as to strengthen the traversing capability of the VSG under grid-connected fault;
(3) The self-adaptive current limiting method provided by the invention can automatically adapt to the voltage drop value without setting an additional fault ride-through mode, so that uncertainty between VSG control output quantity and an actual measured value during mode switching is avoided;
(4) The simulation result shows that the PV-ESS-VSG fault ride-through method taking the dynamic self-adaptive current limiting of voltage drop into account has good ride-through and current limiting capability in balanced and unbalanced voltage drop faults.
It is noted that relational terms such as first and second, and the like are used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Moreover, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one … …" does not exclude the presence of other like elements in a process, method, article, or apparatus that comprises the element.
Although embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made therein without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.
Claims (5)
1. The self-adaptive virtual admittance current limiting method based on the voltage drop dynamic is characterized by comprising the following specific steps of:
1) When the grid connection point fails, a fault detection signal Will be set to 1 from 0;
2) The power angle and the virtual potential generation layer output are locked to rated values;
3) Calculating an active power reference value and a reactive power reference value, and substituting the active power reference value and the reactive power reference value into a quasi-static control mathematical model to obtain a virtual resistor and a virtual inductor;
4) Updating the virtual resistor and the virtual inductor obtained in the step 3) in the current control model;
5) Grid-connected point fault clearing and fault detection signals Setting 0 from 1;
6) Fault signal The falling edge triggers the reset integrator I 1, and updates the initial value of the reset integrator I 1 to output the electric angle from the synchronous control layer at the moment, and the output of the virtual potential generating layer is unlocked;
7) Repeating the steps 3) and 4), and updating the virtual resistor and the virtual inductor to steady-state values.
2. The adaptive virtual admittance current-limiting method based on voltage drop dynamics according to claim 1, wherein the method comprises the following steps: the power reference value equation to be compensated during the fault period in step 1) is:
（1）
Wherein the method comprises the steps of And/>Respectively the active power and reactive power reference values which should be output when in fault,/>And/>For rated apparent power and its maximum value, the maximum value is 1.1 times of rated value; /(I)Grid-connected point voltage per unit value,/>And/>Respectively negative sequence components of grid-connected point voltages.
3. The adaptive virtual admittance current-limiting method based on voltage drop dynamics according to claim 1, wherein the method comprises the following steps: the formula for calculating the active power reference value and the reactive power reference value in the step 3) is as follows:
（3）
Wherein the method comprises the steps of And/>The virtual resistor and the virtual inductor are respectively used for preventing the switching tube from being damaged due to the fact that the switching tube passes through large current during the transient state of the converter, adapting to different power grids and providing power support, and according to the rule of grid-connected standards, outputting reactive current is prioritized during the fault period, and the reference value of the active current can be calculated through the annular current limiter so as to ensure that the inverter is not excessively flowed;
（4）
In the above And/>Are all current inner loop reference values,/>Output rated current per unit value for positive sequence,/>For the per unit value after grid-connected point voltage drop,/>For the maximum current value that the converter can bear under a certain safety margin, take/>Is that；
Reactive power compensation coefficient of converter during faultAnd/>Is defined as:
（5）
maximum apparent power of network-side converter during fault According to rated mains voltage/>Maximum current that can be transmittedAdjustment is made, which is expressed as:
（6）
（7）
Wherein the method comprises the steps of For positive sequence rated apparent power, when asymmetric voltage drop occurs at the network side, the converter outputs active power/>And reactive/>The expression is:
（8）
、/> and/> 、/>The active and reactive output values of the VSG and the positive sequence reference value of the VSG are respectively obtained;
Wherein:
（9）
in the formula (9), the amino acid sequence of the compound, Is the direct current component, the secondary cosine component and the secondary sine component of the active power,/>Is a direct current component, a secondary cosine component and a secondary sine component of reactive power,/>，/>Network side voltage and current positive sequence/>, respectivelyShaft sum/>Axis component,/>，/>Negative sequence of network side voltage and current/>, respectivelyShaft sum/>The axial component, positive and negative sequence components are extracted according to a trap, and the transfer function of the trap is as follows:
（10）
In the above Is a differential operator,/>Is the quality factor of the trap, and the value of the quality factor is related to the specific design of the trap effect.
4. A voltage sag dynamic-based adaptive virtual admittance current-limiting method according to claim 3, wherein: when the grid side fails asymmetrically, the formula (8) can know that the active power and the reactive power of the converter injected into the power grid have double frequency fluctuation, and the 4 input quantities in the formula (9) are difficult to realize the control of 6 output quantities, so that the two free quantities are divided into two control modes, namely balance current control and power constant control, wherein the balance current control is used for inhibiting negative sequence current components of the converter output and improving the current balance capacity of the grid side, the power constant control is used for inhibiting the double frequency fluctuation of the power grid power, the reactive power double frequency fluctuation is not considered, and in addition, the negative sequence current command under the asymmetric voltage drop is inhibitedThe power reference value is associated with the d, q axis currents for the purpose of:
（11）
In combination with (3), (7) and (11) virtual admittance during failure can be obtained:
（12）
In the above And/>The updated values of the virtual resistor and the virtual inductor under the current limiting strategy in the step 4);
further can obtain
（13）
As can be seen from equation (13), injecting positive-order active current and compensating positive-order reactive current based on the voltage sag level during the fault actually becomes to directly adjust the magnitude of the virtual admittance, and this manner of dynamically adaptively adjusting the virtual admittance can effectively suppress transient current during the fault, while suppressing the rush current caused by the switching of the fault modes.
5. The adaptive virtual admittance current-limiting method based on voltage drop dynamics according to claim 4, wherein: the virtual admittance update value formula in the balance current control is as follows:
（2）
In the middle of And/>Updated values of virtual resistance and virtual inductance under fault period limit respectively,/>And/>The d-axis and q-axis components of the excitation electromotive force,/>, respectivelyAnd/>And the values are the per-unit values of the grid-connected point voltage positive sequence components respectively.
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